As power supply circuits for small electronic appliances such as cell phones, portable information terminals (PDA), note-type personal computers, digital cameras, etc., switching regulators (DC-DC converters) suffering less power loss in voltage conversion are widely used. Passive elements such as inductors, capacitors, etc. used in DC-DC converter circuits should be made smaller to reduce an area occupied by the power supply circuit.
Because the switching frequencies of DC-DC converters have been increased to improve their power efficiency and performance, inductors and capacitors contained in them have smaller constants, making it possible to miniaturize these parts. Thus, the inductors have been changed from a conventional coil type to a laminate type. A laminated inductor is produced by integrally laminating a magnetic sheet or paste of soft ferrite with a conductive paste comprising a high-conductive metal or alloy of Ag, Cu, etc. for forming internal electrodes (conductor patterns), sintering the resultant laminate, printing or transferring a paste for external electrodes on the sintered body, and baking the resultant electrodes.
The DC-DC converters need inductors having stable inductance even at a high frequency or in a strong magnetic field, and excellent DC superposition characteristics. In some cases, the inductors are required to have non-linear inductance to DC current.
With respect to DC superposition characteristics, it is required that soft ferrite used for inductors is not easily saturated even in a strong magnetic field, namely has a high saturation magnetic flux density Bs. Mn—Zn ferrite is known as soft ferrite having high Bs, but its low electric resistance makes it unsuitable for lamination. Accordingly, Ni—Zn ferrite, Ni—Cu—Zn ferrite, Mg—Zn ferrite, etc., which have high electric resistance despite lower Bs than Mn—Zn ferrite, are used.
The laminated inductors suffer several problems. The first problem is that the magnetic permeability of ferrite changes when strain is added. Such phenomenon is called “magnetostriction.” Main factors of applying strain to the ferrite are (a) compression stress generated by the curing shrinkage of a resin in a resin molding, (b) stress generated by the difference in a linear thermal expansion coefficient between an inductor and a printed circuit board, and (c) internal stress generated by the difference in a linear thermal expansion coefficient between the ferrite and an internal electrode metal. With respect to the linear thermal expansion coefficient, the ferrite is about +10 ppm/° C., and Ag is about +20 ppm/° C.
Internal stress in the laminated inductor deteriorates the magnetic characteristics such as inductance and quality coefficient (Q value) of ferrite, and heat shock in a soldering step, etc. generates cracks in the device. As a result, the laminated inductor has uneven performance and low reliability.
To suppress the variation of characteristics due to magnetostriction, JP 8-64421 A proposes a laminated inductor, in which carbon paste layers between magnetic layers are caused to disappear to form void layers for stress relaxation. However, it has been found that the formation of void layers is not sufficient for stress relaxation, and that voids lower the strength of the laminated inductor. In addition, a gas generated during eliminating the carbon paste causes the delamination (peeling of layers) of the laminated inductor and the cracking of ferrite. The delamination and cracking are likely to permit a plating liquid, etc. to intrude into the laminated inductor, causing the short-circuiting of conductor patterns.
JP 56-155516 A proposes an open-magnetic-path-type inductor comprising non-magnetic, insulating layers interposed between magnetic layers to have magnetic gaps in a magnetic circuit for improving DC superposition characteristics. However, JP 56-155516 A does not consider the variation of magnetic properties by internal stress at all. In addition, because the non-magnetic, insulating layers reach an outer surface of this inductor, a plating liquid, etc. are likely to intrude into the inductor through cracks and delaminated portions in interfaces between the magnetic layers and the non-magnetic insulating layers, causing the short-circuiting of conductor patterns.